This invention relates to a color cathode ray tube (CCRT) having an arc suppression structure to minimize surge currents from the CCRT caused by internal arcing, and more particularly relates to such a CCRT having a composite internal conductive coating with arc suppression features.
Co-pending U.S. patent application Ser. No. 525,758, filed Aug. 28, 1983, describes and claims a CCRT having certain feedback features making such tube suitable for use with high resolution color displays, having an automatic convergence feature.
CCRT's are known to be susceptible to occasional internal arcing due to their high operating potentials (typically 25 to 30 kilovolts) and small inter-electrode spacings (typically 30 mils). Such arcing results in momentary surge currents up to several hundred amps, high enough to destroy critical components in the external circuitry.
The feedback CCRT of the above-mentioned co-pending Application incorporates certain arc suppression features including a high resistance internal conductive coating in the neck region adjacent the high potential electrodes of the electron gun, and a getter structure designed to result in an effective getter "flash", a deposit of gas-adsorbing material essential to adequate life, without the formation of a conductive bridge across the coating.
High resistance coatings in the neck region are known to be effective "surge limiters" by suppression of arc currents during tube operation. See for example, U.S. Pat. Nos. 2,829,292; 3,555,617; 3,961,221; 3,959,686; 4,249,107; 4,280,931 and German Pat. No. 2,634,102. However, such coatings can hinder high voltage conditioning during tube processing. In high voltage conditioning, a voltage of 40 kilovolts or more is applied between the terminal high voltage electrode and adjacent electrodes of the electron gun to remove projections and foreign matter from the interelectrode spacings. Electrical contact with the high voltage electrode is made by contacting the external surface of the anode button, a high voltage contact which protrudes through a forward portion of the funnel wall to make contact with the internal conductive coating. This coating extends into the neck region, where the circuit is completed by spring-like snubbers, attached to the terminal portion of the gun and making contact with the internal conductive coating.
The snubbers, typically spoon-shaped, and fabricated of spring steel, are usually arranged with the bowl-shaped contact portion extended beyond the terminal portion of the gun. Typically, three such snubbers are attached equidistantly about the periphery of the top cup or convergence cup of the gun. During insertion of the gun into the tube, the snubbers must be deflected inward slightly to clear the neck wall. The bottoms of the bowl-like portions make low-friction sliding contact with the internal coating as the gun is moved into position. Once in position, each snubber makes a firm, essentially point contact with the coating.
During normal tube operation, the current across these point contacts is almost zero. Occasional spurious currents are on the order of a few microamps. However, during high voltage conditioning, the current across these contacts reaches peaks of the order of tens of amps. While the conditioning energy is pulsed, these peak currents are nevertheless apparently of sufficient duration and intensity to cause erosion of the high resistance coating in the area immediately under the contact. In severe cases, such erosion leads to loss of contact and, consequently, tube failure.
It has been suggested to alleviate this erosion problem by placing a lower resistance coating between the snubber and the high resistance coating, thus eliminating the troublesome point contact with the high resistance coating, and providing a larger area of contact through the lower resistance coating. However, this arrangement reduces the effectiveness of the arc suppression feature of the high resistance coating by providing a low resistance path between the coating and the gun, effectively shorting out a significant portion of the arc suppression coating.